1. Field of the Invention
The present invention relates to a high-voltage semiconductor device which develops a stable breakdown voltage characteristic.
2. Description of the Related Art
Conventionally, a high-voltage diode and a MOSFET have been used as a semiconductor device for electric power control. Such a kind of semiconductor device has been required to improve conflicting characteristics, such as, an improvement in breakdown voltage and a reduction in on-state resistance, and a variety of proposals on this requirement have been offered.
For example, JP-A H9-191109 (KOKAI) discloses a technique which disposes a plurality of stripe-shape p-type buried layers near by the surface of an n-type base layer of a Schottky barrier diode (FIG. 12 of the patent document). This p-type buried layer is designed such that a depletion layer from a Schottky interface reaches the p-type buried layer before the electric field at the Schottky interface reaches maximum electric field of the semiconductor surface.
After that, as raising the reverse bias voltage, the depletion layer reaches the p-type buried layer, the largest point of the electric field at the Schottky interface is fixed not to be raised, and the depletion layer newly spreads onto an ohmic electrode side from the buried layer. Here, the voltage held by the epitaxial layer between the Schottky interface and the buried layer is set to a value lower than the maximum blocking (breakdown) voltage. Thereby, the electric field at the Schottky interface is fixed to a low value, and the leakage current from the Schottky interface may be decreased.
The technique disclosed in the aforementioned patent document disposes a plurality of stages of p-type buried layers as depicted in FIG. 13 of said patent document, shares a breakdown voltage by means of n-type base layers divided by the plurality of stages of p-type buried layers, and develops a Schottky barrier diode which achieves a small voltage drop when conducting at a high voltage.
However, the forgoing conventional improvement is specified only for an active region under an anode electrode, and it does not take so much account to an n-type base layer under an edge termination structure. In other words, specifically, a planer-type high-voltage device, though it has high-voltage performance based on the p-type layer buried in the active region, it poses the problem that the breakdown voltage is determined by an electric field concentration at an edge termination region. If the edge termination structure is microscopic like guard rings, and when widths and intervals of the patterns vary due to a mask misalignment, the problem of lowering in breakdown voltage occurs.
Although such a problem poses, the planer device has many advantages for mass production on those points which are more stable in process, and are higher in yield ration and in throughput than a mesa-type device. Therefore, it has been desired to attain a high-breakdown-voltage device having a planar structure stable in breakdown voltage.